The present invention relates to a perfected piston pump, in particular a radial-piston pump for internal combustion engine fuel.
As is known, in internal combustion engines, and particularly diesel engines, pumps of the above type operate at high pressure of even over 1300 bar, and at high speed of about 3000 rpm.
The cylinders of a piston pump of the above type are arranged radially about a drive shaft comprising an eccentric portion on which rotates a cam for operating the cylinders, each of which comprises a pad engaging a respective flat portion of the cam. In use, the eccentric portion moves the cam, parallel to itself at all times, along a circular trajectory, so that each pad slides on the respective flat portion with no angular oscillation of the axis of the respective piston.
The shaft, eccentric portion, cam and pads of the pump are housed inside a closed chamber into which part of the fuel supplied to the pump is fed to lubricate the surface of the pad contacting the flat portion of the cam; and sliding bearings, also lubricated by the fuel circulating in the chamber, are provided between the shaft and respective seats, and between the cam and the eccentric portion.
Over and above a given pressure on the movable contacting surfaces and/or over and above a given speed of the pump, however, lubrication by the fuel or any type of lubricating oil is insufficient to lubricate the pad surfaces and respective flat portions of the cam. Such pressure, in fact, compresses the film of fuel or oil between the contacting surfaces, so that it is expelled from the respective gap, thus resulting in possible seizure of the two surfaces.